Those freed on Wednesday included two Americans, two Frenchmen, two Indonesians and a Canadian taken from an Afren oil rig, eight Nigerians abducted from an Exxon Mobil platform and four others, security sources involved in the operation said.

"It was a land, air and marine assault. They have all been freed, all 19 of them," one of the sources told the Reuters news agency, asking not to be named due to the sensitive nature of the operation.

The victims were all taken hostage in recent raids on oil facilities in the southern region.

'Rebel co-operation'

A security source involved in planning the operation told Reuters that a former leader of one of the group's fighting in the delta had been instrumental in securing the safe release of the captives.

"There was a strong element of co-operation between the former militant leadership and the security forces in the release of the 19 hostages," the security source was quoted as saying.

"What happened today in Nigeria has never happened before. It shows the strength not only of the security forces but of the former militant leadership and the amnesty programme."

Several field commanders of the Movement for the Emancipation for the Niger Delta (Mend) and other groups agreed to lay down their weapons last year after an amnesty was offered by the government.

Thousands of their fighters are said to have followed suit, in return for a presidential pardon, regular payments, and the promise of retraining.

The amnesty was credited with alleviating the security problems which had seen oil output in the Niger delta plummet in recent years.

But many observers say that the pardons and payments have failed to address the underlying causes of the unrest. Mend claims to be fighting for a fairer distribution of oil revenue in an area of poverty and high unemployment.

'Happy ending'

France's foreign minister confirmed that two of its nationals were among those released.

"This happy ending does not make us forget the fate of the other French nationals still held hostage, for which the government is as mobilised as ever," Michele Alliot-Marie said.

France has eight nationals held overseas including five in Mali, two in Afghanistan and one in Somalia.